This invention relates to an injector for injecting a traceable material or a tracer into a bore hole that is connected to an oil and/or gas reservoir.
When a promising reservoir of oil and/or gas is discovered, the following procedure is used to determine the size and shape of the reservoir. The field is divided into a number of geometrical squares, followed by the performing of a drilling operation to obtain bore holes in the squares. In some of the bore holes there are placed injectors according to a certain pattern, and at various depths. A traceable material is then injected from the injectors into the oil and/or gas reservoir, followed by measuring the amount of distribution of tracer or tracer elements. This measurement is performed by providing an amount of detectors placed in near and distantly located bore holes according to a pre-calculated geometrical pattern. The tracer injections may be repeated after a certain time, at intervals, in dependence on the content of the reservoir, the permeability, the temperature and pressure, and finally the properties of the traceable material.
Two different arrangements for injecting a traceable material in an oil and/or gas reservoir have been known for a long time. One arrangement comprises the provision of an explosive charge in relation to a piston in the injector. The charge may for instance be detonated by means of a timer, whereby the piston compresses a container of traceable material, followed by the injection of the contents into the reservoir.
The other arrangement, e.g. as described in U.S. Pat. No. 4,220,264, comprises a hydraulic system with a piston, manually actuated by the provision of a valve/spring device that compresses a container of traceable material, followed by the injection of the traceable material into the reservoir.
The above mentioned arrangements are encumbered with considerable disadvantages. One disadvantage that occurs when using a timer, or when the depth level is indicated by measuring the wire length that is paid out, is that the calculated depth level may be encumbered with considerable calculation errors since the lowering speed may vary, caused by speed variations in the winch motor, strain in the wire, and finally the lowering operation itself may be disturbed by faults. Further, it is difficult to estimate the most favorable charge for a satisfactory distribution of the traceable material.
Another disadvantage in injection of the traceable material into the reservoir is that the annulus defined between the injector and the wall in the bore hole will not be sealed. As a result of this, the precise depth level of injection may not be exactly determined, because the traceable material is allowed to distribute lengthways in the bore hole before entering the reservoir. Consequently, the calculations of such reservoir parameters as the amount of oil and gas, the depths and the dissemination will be encumbered with statistical uncertainty.
A further disadvantage is that over a period of time, tracer material will accumulate in and upon the injector. When applying a radioactive material, for instance cesium, repeated handling of the injector may cause health injuries to the operators.
It is therefore important that the injector has provisions to be cleaned after the injection, and that the depth level of the bore hole may be exactly determined to measure the distribution of the tracer, and accordingly the character of the reservoir can be calculated very precisely.